This invention relates generally to an optical information recording medium and, more specifically, to a WORM-type recording medium which can write, only once, with a laser beam and which can repeatedly read the written information with a laser beam.
One known WORM-type compact disc includes a substrate, a light absorption layer obtained by spin coating an organic dye, such as a cyanine dye, on one surface of the substrate, and a metal reflective layer provided on the light absorption layer (Japanese Published Unexamined Patent Applications Nos. 2-42652 and 2-132656). The feasibility of the spin coating permits mass-production of the compact discs and this technique is economically quite advantageous. The known compact disc has, however, a drawback that the service life thereof is relatively short because the cyanine dye is susceptible to a chemical change by sunlight.
There is also a known WORM-type compact disc using a phthalocyanine compound as an organic dye for the light absorption layer (Japanese Published Unexamined Patent Applications Nos. 58-183296 and 58-37851). The phthalocyanine dye-containing compact disc shows better light resistance in comparison with the cyanine dye-type compact disc. However, the phthalocyanine dye-type compact disc fails to simultaneously satisfy both optical characteristics required for writing optical information and a high index of reflectance required for reading the written information. In particular, the known phthalocyanine-type compact disc has a low absorbancy index (absorbancy per unit thickness of the light absorption layer) at a wave length near that of the laser beam used for the writing and reading, so that optical information cannot be satisfactorily written or recorded on the disc.